High density bimodal polyethylene compositions, and in particular, high density “bimodal.” or “multimodal” polyethylenes (“bHDPE”), are known to be useful in making films suitable for a variety of commercial products such as films, pipes, blow molding, etc. However, the costs of producing such compositions is a disadvantage—being relatively high—as most bHDPEs are produced in two stages or more, and/or in two or more staged reactors such as the processes of Dow, Basell, Borealis and Mitsui. Such commercial polymerization systems are reviewed in, for example, 2 METALLOCENE-BASED POLYOLEFINS 366-378 (John Scheirs & W. Kaminsky, eds. John Wiley & Sons, Ltd. 2000).
Further, the processing of bHDPEs can present further commercial problems. For example, it is known that film cooling, upon extrusion of the polyethylene, is a limiting factor in film production, especially for extrusion of high density polyethylene, such as described in FILM EXTRUSION MANUAL, PROCESS, MATERIALS, PROPERTIES, pp. 497 (TAPPI, 1992). One solution to this problem is to operate at a desirably low melt temperature. However, given the bimodal nature of these resins, melting may be uneven, and/or relatively high melt temperatures must be maintained for the given resin. To compensate, high back pressures can be maintained, but this can lead to other problems, and consumes more energy. What would be desirable is a bHDPE that can be extruded at a rapid rate at a relatively low melt temperature, using lower extruder motor loads, while maintaining high film quality.
As a further advantage, it would be desirable to use a low cost process to produce bHDPE. Single reactor systems may offer such a cost advantage. While single reactor systems have been described as capable of producing bimodal polyethylenes for film applications, such as described by H.-T. Liu et al. in 195 MACROMOL. SYMP. 309-316 (July, 2003), those films must still match the quality and processability of current dual-reactor derived polyethylene films for commercial viability. The present invention in one aspect is directed towards such a film, as the inventors have found that a certain balance of polymer properties can meet these commercial needs to produce polyethylene films suitable for cast, blown and other film products; and further, that it is possible to achieve these ends using single-reactor produced polyethylene compositions.